As mobile devices have been continuously developed and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased. Among these secondary batteries, lithium secondary batteries, which exhibit high energy density and operating voltage and excellent charge retention and service-life characteristics, have been widely used as an energy source for various electronic products as well as various mobile devices.
Depending upon the kind of external device in which a secondary battery is used, the secondary battery may be configured to have a detachable type structure in which the secondary battery can be easily inserted into and removed from the external device or to have an embedded type structure in which the secondary battery is embedded in the external device. For example, the detachable type secondary battery can be inserted into or removed from devices, such as laptop computers, as needed. On the other hand, devices, such as some kinds of mobile phones, MPEG Audio Layer-3 (MP3) players, tablet PCs, and smart pads, require an embedded type battery pack due to the structure or capacity thereof.
Meanwhile, various kinds of combustible materials are contained in a lithium secondary battery. As a result, the lithium secondary battery may be heated or may explode in the event of the overcharge of the lithium secondary battery, overcurrent in the lithium secondary battery, or the application of some other external physical impact to the lithium secondary battery. That is, the safety of the lithium secondary battery is very low. For this reason, safety elements, such as a positive temperature coefficient (PTC) element and a protection circuit module (PCM), which are capable of effectively controlling the abnormal state of the lithium secondary battery, such as the overcharge of the lithium secondary battery or the overcurrent in the lithium secondary battery, are connected to a battery cell of the lithium secondary battery.
Preferably, such a secondary battery is manufactured so as to have as small a size and weight as possible. For this reason, a prismatic battery or a pouch-shaped battery, which has a small weight to capacity ratio, is usually used as a battery cell of the secondary battery. In particular, much interest is currently focused on a pouch-shaped battery that uses an aluminum laminate sheet as a sheathing member because such a pouch-shaped battery is lightweight and the cost of manufacturing the pouch-shaped battery is low.
In the case in which a battery pack is manufactured using a pouch-shaped battery cell, as described above, a PCM is connected and fixed to electrodes of the battery cell in the state in which the battery cell is mounted to a case, and a label is attached to the outer surface of the case, whereby the battery pack is manufactured. For example, the case may include an upper cover and a lower cover, which are coupled to each other in order to surround the outer surface of the battery cell. Alternatively, the case may be configured to have a frame structure for fixing the outer edge of the battery cell.
However, the necessity for a battery cell to have a compact structure and exhibit improved structural stability has increased. As a result, there has been used a method of surrounding the outer edge of the battery cell through a hot-melt process instead of the frame.
Specifically, FIG. 1 is a sectional view schematically showing the structure of a mold used in a conventional hot-melt process.
Referring to FIG. 1, a battery cell 130 is placed in a mold 110, and a hot-melt resin is injected into the mold 110 through injection ports 120 formed in the upper and lower parts of the mold in order to wrap the outer edge of the battery cell with the resin.
Subsequently, the battery cell 130, the outer edge of which has been wrapped with the resin, is removed from the mold 110, and then, although not shown, a protection circuit board and a PCM received in a PCM case, to which the PCB is mounted, are loaded on the outer surface of a receiving part of the battery cell and a thermally welded surplus portion (i.e. a terrace) of the battery cell using a piece of double-sided adhesive tape, whereby a battery pack is assembled.
In the case in which the above method is used, however, the hot-melt resin injected through the injection ports is not distributed to portions of the mold that are distant from the injection ports, whereby the defect rate of molded products is increased. When high pressure is applied to the hot-melt resin in order to distribute the resin to the portions of the mold that are distant from the injection ports, a high-pressure impact is applied to the battery cell.
In addition, in the case in which the PCM is mounted using the double-sided adhesive tape, the PCM easily becomes unstable in response to external impact, and additional process cost is increased due to the use of various parts, such as double-sided adhesive tape, a lower case, and a PCM holder.
Therefore, there is a high necessity for battery pack manufacturing technology that is capable of reducing the defect rate in the process, improving the durability of battery packs, reducing the cost of manufacturing the battery packs, and maximizing the capacity of the battery packs.